The present invention relates to a filter for filtering and suppressing transient surge and impulse voltages and currents on power lines and, in particular, to a filter for instantaneously filtering impulse noise signals applied to a power line from such sources as lightning, nuclear explosions and large equipment switching surges in order to protect sensitive electronic equipment connected to the power line.
Communications equipment, computers, military targeting systems, home stereo amplifiers, televisions and other electronic devices are increasingly characterized by small electrical contacts and miniature components which are very vulnerable to interference or damage from stray electrical energy carried by power lines. Unpredictable variations in power line voltage changes the operating range and can severely damage or destroy such electronic devices. These electronic devices are very expensive to repair or replace and therefore, require cost effective transient protection.
One source of harmful electrical energy is lightning. Lightning is a very complex electromagnetic energy source having potentials estimated at from five million to twenty million volts and currents reaching thousands of amperes. A lightning stroke generally contains a series of pulses each having a duration of from a nanosecond to several milliseconds. A typical "8/20" lightning pulse lasts for a period of 40 microseconds and has a peak current of 20,000 amps which is reached in 8 microseconds.
Another source of unwanted electromagnetic energy is a nuclear electromagnetic pulse. An electromagnetic pulse generated by a nuclear detonation produces intense transient electric and magnetic fields with very short rise times and a frequency spectrum extending from approximately zero to more than 100 megahertz. The electromagnetic pulse from a high altitude explosion typically has a maximum field strength near the ground on the order of 50 kilovolts per meter, a time duration of one microsecond and a rise time of nanoseconds.
Other sources of impulse noise are switching transients caused by turning on and off large banks of equipment and large motors, and ground loop interference caused by varying ground potentials.
The above-mentioned noise transients can be coupled to a sensitite electronic device by external or internal coupling. In external coupling an electromagnetic wave or a lightning pulse impinges on a receiver, such as a power transmission line or system, and the transient voltage induced in the receiver is passed through transformers, rectifiers and other voltage and current altering devices to the sensitive electronic device. The passage of the transient through such devices as transformers alters the original waveform increasing the current levels, the voltage levels and the frequency spectrum. The characteristics of the transient reaching the sensitive equipment can be hard to predict because of the intermediate coupling devices, thereby making the transient hard to remove . Thus, there is a need in the art for a filtering device that has a broad range of energy absorption and/or shunting capability which responds instantaneously to any type of undesired noise transient.